1. Field of the Invention
This invention relates to a papermaking additive which comprises a specific water-soluble copolymer composed mainly of acrylamide and/or methacrylamide and which can exhibit an excellent effect of increasing paper strength and of retaining a size and a filler in papermaking in a neutral to alkaline pH region.
2. Description of the Prior Art
The acid sheet forming process using alum has previously been the mainstay of the papermaking industry. However, in the acid sheet forming process, the resulting paper is susceptible to degradation and the paper machines undergo corrosion. Moreover, as the papermaking process has recently been carried out in a closed system as a measure of saving energy and controlling environmental pollution, there has been a move toward the employment of a papermaking system with a reduced amount of alum or an alum-free, namely towards a neutral or an alkaline papermaking system. With this move toward higher pH papermaking, it has been desired to develop a papermaking additive which can exhibit a superior effect of retaining a filler and a size and increasing paper strength in papermaking carried out in a neutral to alkaline pH region.
Papermaking additives comprising polymers composed mainly of acrylamide units have been reported so far. The Hofmann rearrangement and Mannich reaction are known as the method of ionizing acrylamide-type polymers. The Hofmann rearrangement product shows a relatively good effect but still has unsatisfactory stability. The Mannich reaction product, when used in papermaking in a neutral to alkaline pH region, decreases in cationic character and does not exhibit a sufficient effect.
The present inventors previously invented a paper strength increasing agent comprising as an active component a terpolymer composed of a water-soluble cationic monomer, an alpha,beta-unsaturated dicarboxylic acid and acrylamide, and applied it for patent (Japanese Laid-Open Patent Publication No. 94697/1985). However, when an acrylic acid ester such as commercially available dimethylaminoethyl methacrylate is used as the water-soluble cationic monomer, the resulting terpolymer is liable to decrease in cationic character or is susceptible to hydrolysis in papermaking system in a neutral to alkaline pH region.